* Add profile to json+ld in Accept

It's required by the ActivityPub spec

* Use headers['Content-type'] instead of mime_type

mime_type strips the profile from the content type, but it's still available raw in the headers hash

* Add test for ld+json with profile

* No need to re-require sidekiq plugins, they are required via Gemfile

* Add derailed_benchmarks tool, no need to require TTY gems in Gemfile

* Replace ruby-oembed with FetchOEmbedService

Reduce startup by 45382 allocated objects

* Remove preloaded JSON-LD in favour of caching HTTP responses

Reduce boot RAM by about 6 MiB

* Fix tests

* Fix test suite by stubbing out JSON-LD contexts

* Added #link_header spec

* Added #call spec

* Delete spec of private methods

* Added #link_header spec

* Added #perform_request spec

Fetching atom extracted from FetchRemoteAccountService and FetchRemoteStatusService
into FetchAtomService. Mentions of the constant "http://activityschema.org/collection/public"
skipped as it's not a real URL/user.